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8.9.4.2 - Ring substitution

Interactive Audio Lesson

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Introduction to Ring Substitution in Aromatic Carboxylic Acids

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Teacher
Teacher

Good morning, everyone! Today we will explore the fascinating world of electrophilic substitution in aromatic carboxylic acids. Can anyone tell me what electrophilic substitution is?

Student 1
Student 1

Isn't it when an electrophile replaces a hydrogen atom on an aromatic ring?

Teacher
Teacher

Exactly! Now, when we talk about aromatic carboxylic acids, how does the presence of the carboxyl group affect this substitution?

Student 2
Student 2

I think it might make the ring less reactive.

Teacher
Teacher

That's correct! The carboxyl group is electron-withdrawing, which decreases the electron density of the ring, leading to deactivation. Thus, electrophilic substitution mainly happens at the meta position. Let's discuss why that is next.

Mechanism of Electrophilic Substitution in Aromatic Carboxylic Acids

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Teacher
Teacher

Now, let's dive into the mechanism of how electrophilic substitution occurs in aromatic carboxylic acids. Can anyone explain what happens during the formation of the arenium ion?

Student 3
Student 3

The electrophile attacks the aromatic ring, forming a temporary arenium ion.

Teacher
Teacher

Correct! This step generates a positive charge on the ring. However, the carboxyl group's electron-withdrawing effect stabilizes this ion, preventing further substitutions at the ortho and para positions. Why do you think those positions are less favorable?

Student 4
Student 4

Because the resonance structures wouldn't stabilize the positive charge well there as they do at the meta position!

Teacher
Teacher

Exactly right! The resonance delocalization of charges in the meta position is much more stable. So, in summary, we see carboxylic acids favor substitution at the meta position due to resonance stabilization. Despite this, they do not undergo Friedel-Crafts reactions.

Applications in Organic Synthesis

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Teacher
Teacher

How do we think understanding the ring substitution of aromatic carboxylic acids can be useful in organic synthesis?

Student 1
Student 1

It might help us in designing compounds with specific properties!

Teacher
Teacher

Absolutely! Being able to predict where substitutions will occur allows chemists to synthesize targeted compounds effectively. What about reactions where aromatic groups are involved? Why do we avoid Friedel-Crafts with carboxylic acids?

Student 2
Student 2

Because the carboxyl group is too deactivating for the Lewis acid catalyst to function effectively.

Teacher
Teacher

Spot on! The interaction between the carboxyl group and the catalyst prevents efficient electrophilic attack. This consideration is crucial in planning synthetic routes. Now, can anyone summarize the main takeaway from our discussion today?

Student 3
Student 3

Aromatic carboxylic acids undergo electrophilic substitution primarily at the meta position due to their deactivating carboxyl groups, and they don't participate in Friedel-Crafts reaction.

Introduction & Overview

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Quick Overview

This section discusses the ring substitution reactions of aromatic carboxylic acids, explaining the deactivating nature of the carboxyl group in electrophilic substitution processes.

Standard

Aromatic carboxylic acids undergo electrophilic substitution reactions where the carboxyl group directs incoming substituents to the meta position due to its deactivating effect. This section requires understanding how the structure of the carboxylic acids influences their reactivity.

Detailed

Detailed Summary

In this section, we delve into the electrophilic substitution reactions of aromatic carboxylic acids. The carboxyl group (-COOH) confers deactivation on the aromatic ring, making it less reactive compared to unsubstituted or activating substituent-affected rings. This deactivation is attributed to the electron-withdrawing nature of the carboxyl group, which stabilizes the negative charge developed during the formation of the arenium ion intermediate. The substitution primarily takes place at the meta position due to the specific resonance structures that dominate the reaction's pathway. As a result, aromatic carboxylic acids do not participate in Friedel-Crafts reactions, which rely on the reactivity of the aromatic ring being preserved. Understanding these mechanisms is crucial when predicting the products of reactions involving aromatic carboxylic acids in organic synthesis.

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Audio Book

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Halogenation Overview

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Carboxylic acids having an a-hydrogen are halogenated at the a-position on treatment with chlorine or bromine in the presence of small amount of red phosphorus to give a-halocarboxylic acids. The reaction is known as Hell-Volhard-Zelinsky reaction.

Detailed Explanation

In this chunk, we focus on the halogenation of carboxylic acids with alpha-hydrogens. The presence of an a-hydrogen allows the reaction to occur at the a-position, where the halogen, chlorine or bromine, can replace a hydrogen atom. The reaction occurs when the carboxylic acid is treated with chlorine or bromine in the presence of red phosphorus, which acts as a catalyst. This process leads to the formation of a-halocarboxylic acids, meaning that a halogen replaces the hydrogen atom attached to the a-carbon relative to the carboxyl group. This specific reaction is called the Hell-Volhard-Zelinsky reaction, which is significant in synthetic organic chemistry, allowing for the introduction of halogen functionalities into carboxylic acids.

Examples & Analogies

Think of the a-hydrogen in the carboxylic acid like a puzzle piece that can be swapped out. In the puzzle of creating complex molecules, sometimes we want to upgrade a piece (by adding a halogen) to improve the overall picture (the functionality of the molecule). For example, the chlorine could be considered a modern touch to an older puzzle piece to enhance its value and functionality in various chemical reactions.

Electrophilic Substitution Reaction in Aromatic Carboxylic Acids

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Aromatic carboxylic acids undergo electrophilic substitution reactions in which the carboxyl group acts as a deactivating and meta-directing group. They however, do not undergo Friedel-Crafts reaction (because the carboxyl group is deactivating and the catalyst aluminium chloride (Lewis acid) gets bonded to the carboxyl group).

Detailed Explanation

Aromatic carboxylic acids, due to the presence of the -COOH group, participate in electrophilic substitution reactions. However, this group is a deactivating group, which means it makes it less reactive towards electrophiles compared to other substituents, such as alkyl groups. When these carboxylic acids undergo electrophilic substitution, they direct new substituents to the meta position relative to the carboxylic acid group instead of the ortho or para positions. This is crucial to understand because it influences how products are formed in the reactions. Also, aromatic carboxylic acids are not suitable for Friedel-Crafts acylation reactions due to similar deactivation; the Lewis acid catalyst tends to bond with the carboxyl group, preventing proper substitution reactions from occurring.

Examples & Analogies

Consider trying to invite guests to a party (electrophiles) while your friend (the carboxyl group) stands guard at the entrance. If your friend is more of a strict or busy guard (deactivating), they might only let guests in through the side door (meta position) rather than the front (ortho or para). This means that only certain guests can come in and hang around, keeping the party focused on specific interactions instead of allowing a chaotic mix of attendees (substituents).

Definitions & Key Concepts

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Key Concepts

  • Electrophilic substitution involves an electrophile replacing a hydrogen atom in the aromatic ring.

  • The deactivating carboxyl group directs substitution to the meta position.

  • Arenium ions are formed as intermediates during these substitution reactions.

  • Carboxylic acids do not undergo Friedel-Crafts reactions due to the deactivation of the ring.

Examples & Real-Life Applications

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Examples

  • When benzene-1,2-dicarboxylic acid reacts with bromine, bromination occurs at the meta position instead of the ortho or para positions.

Memory Aids

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🎵 Rhymes Time

  • Carboxyls pull charge away, meta substitutions are here to stay.

📖 Fascinating Stories

  • Imagine a party where the cool kids, the electrophiles, want to join the ring of friends, but the carboxyls only invite them to the meta seat due to their deactivating nature.

🧠 Other Memory Gems

  • CAMP: Carboxylic acids Always Meta Preference.

🎯 Super Acronyms

CARB

  • Carboxyls Affect Reactivity Bodily (in the meta position).

Flash Cards

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Glossary of Terms

Review the Definitions for terms.

  • Term: Electrophilic Substitution

    Definition:

    A reaction where an electrophile replaces a hydrogen atom in an aromatic ring.

  • Term: Carboxyl Group

    Definition:

    A functional group (-COOH) consisting of a carbonyl and a hydroxyl moiety.

  • Term: Arenium Ion

    Definition:

    A positively charged intermediate formed during the electrophilic substitution of an aromatic compound.

  • Term: Meta Position

    Definition:

    The carbon atom in an aromatic ring that is two carbons away from a substituent.

  • Term: FriedelCrafts Reaction

    Definition:

    A type of electrophilic aromatic substitution that involves adding an acyl or alkyl group to an aromatic ring using a Lewis acid as a catalyst.